Metaplasia, as documented in vertebrates (e.g. Yamada, 1967; Steen, 1971), is difficult to study in complex systems where cell transformations are not synchronous, many cell types are present, and cell movements complicate the task of tracing cell lineages. Thus it would be preferable to study these transformations in a previously defined, analogous system where these factors can be eliminated. Such a model system is present in the insect Manduca sexta. Simple tubular epithelial glands function as silk glands in the catepillar and salivary glands in the adult portions of the M. sexta life cycle. A single, uniform population of differentiated silk gland cells transform synchronously into four differentiated cell populations of the adult gland, without cell division, death, or large scale morphogenetic movement. Using this system, the objective will be to study the timing of adult cell determination and the process of cellular redifferentiation. (1) Surgical modification and transplantation of larval glands to new hormonal milieus will be used to determine when these larval glands are programmed, and over what length of time the programming proceeds, limiting a gland cell's adult fate. (2) Combined structural and biochemical studies will be used to follow the process of cellular redifferentiation in a precise temporal framework. (3) The timing of DNA synthesis in development will be followed by labeling and ploidy studies. The effects of inhibition of DNA synthesis on adult gland development will then be followed. (4) Finally, in transplant experiments, the ability of larval cells to bypass a portion of their larval life cycle will be followed by structural and H3-thymidine uptake observations.